To the present, in the medical and printing-plate making fields, effluent resulting from wet processing of image forming materials has become problematic in terms of workability, and in recent years, from the view point of ecology as well as space saving, a drastic decrease in processing effluent has been increasingly demanded. Accordingly, photothermographic materials, which form images only via heat, have been brought to practicale and are rapidly prevailing in the above fields.
A photothermographic material (hereinafter, also referred to as a thermally developable material or a photosensitive material) itself has been proposed already for many years and is described, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075.
This thermally developable material is generally processed via a thermal development apparatus, which is known as a thermal developing device and stably heats a thermally developable material to form an image. As described above, in accordance with its rapid popularity in recent years, many thermal development apparatuses have come on the market. Further, in recent years, a compact laser imager and rapid processing have been desired.
For the above reason, characteristic improvements of a thermally developable material is essential. To obtain a sufficient image density even during rapid processing, it is effective to increase the covering power due to increased number of coloring points by employing silver halide provided with a smaller mean particle diameter as disclosed in JP-A Nos. 11-295844 and 11-352627 (hereinafter, JP-A refers to Unexamined Japanese Patent Application Publication); to utilize a highly active reducing agent provided with a secondary or tertiary alkyl group as disclosed in JP-A No. 2001-209145; and to utilize a development accelerator such as hydrazine compounds or vinyl compounds, and phenol derivatives and naphthol derivatives, as disclosed in JP-A Nos. 2002-278017 and 2003-66558.
Further, as means for rapid processing from the hardware aspect, disclosed is a technique in which a thermally developable material is exposed while being heated on a thermal development drum, or is thermally developed simultaneous during the exposure (JP-A Nos. 10-115889, 2002-162692 and 2004-85763).
However, in the case of simultaneous exposure and thermal development, a problem exists of vibration with exposing section being transffed to the thermal developing section due to proximity of the exposing section and the thermal developing section, or resulting in a time lag between the loading and the trailing end of the photosensitive sheet material with respect to time from exposure to thermal development, resulting in generation of uneven density at the time of thermal development (refer to Patent Document 1). Further, observed was uneven density due to variation of the coated layer thickness or coating application.
In addition to more rapid processing, recently, to achieve high image quality, a dry film specifically suitable for digital mammography diagnosis has been desired. For the mammography diagnosis, it is essential that no image defects due to uneven density or foreign matter particles are generated as well as to providing high maximum density, a wide range from the minimum density to the maximum density, and excellent sharpness. However, when the maximum density is increased, in the case of conventional photosensitive materials, particularly those prepared by coating a water-based coating composition, problems in the tone of higher density portions shifts from pure black to reddish, countermeasures of which have been sought. On the other hand, to overcome mottling defects (coating defects), generated at the time of coating, become noticably visible after thermal development, one countermeasure is in which a radiation absorptive substance such as an acutance dye is contained in photosensitive materials to achieve a density of 0.6-3.0 at the exposure wavelength (please refer to Patent Document 2).
However, even utilizing these countermeasures, it is insufficient to prevent uneven density at the time of thermal development nor to improve silver tone at higher density portions, resulting in insufficient desirable characteristics, particularly as a dry film for mammography.
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 2004-138724
Patent Document 2: JP-A 2003-195450